Getting the Most from your 3 Jaw and 4 Jaw Chucks
By: Gord Thorogood
Chucks are usually supplied in a plain back configuration to be mounted onto a backing plate that configuration to the spindle nose of each individual machine. The backing plates are either supplied with the machine or are available from the supplier.
The backing plate is machined to accept the new chuck by mounting the backing plate on the lathe for which the chuck is purchased, and turning it to the proper dimensions. This ensures that the chuck will run true on this machine only. If a chuck is already mounted on a backing plate, there is a chance it will need adjusting if the plate was not machined as above. To bring the chuck true, simply separate the chuck and backing plate and remachine the plate.
Three jaw chucks are usually of the scroll type and the jaws are self centering. In order to keep these units accurate, close tolerances are required on all the moving parts which necessitates that normal preventive maintenance be performed. If the jaws are extended to hold layer pieces and the scroll is exposed cuttings are allowed to enter the chuck and can cause the chuck to lock up. If the chuck is disassembled periodically to clean and inspect, it will give good long service.
Four Jaw chuck usually have independent jaws to hold irregular shaped pieces or turn pieces off centre. The screws and "T" handles of chucks can have a mechanical advantage of up to 90 to 1, and allows tremendous pressure to be exerted on the work and the chuck bodies. The temptation is to over tighten, "just to be sure" which can harm the work and seriously damage the chucks.
Chuck bodies are usually a fine cast iron and can be distorted through the thinnest section because tons of pressure can be exerted and if over tightening is persistent the bodies can fracture and even fly apart.
Three jaw scroll chucks have the jaws centered with the body as a unit, therefore the jaws and the body are numbered as a set. This means that the jaws cannot be interchanged with any other unit and if the jaws are damaged they cannot be exchanged.
Care must be exercised when using the four jaw chuck. Do not use excessive speed when turning pieces off center as the imbalance can cause extreme vibration and possibly even do damage to the machine.
The larger the chuck the more centrifugal forces are generated so remember, just because the machine is capable of high speeds does not mean that all chucks or faceplates can safely be used at these speeds.
Like any machine part, chucks require care in use and common sense maintenance to give long satisfactory service.
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